Printed circuit boards and other electrical and/or electronic devices and/or components are often encapsulated, overmolded and/or underfilled with a polymeric material to provide protection against thermal cycling, moisture and/or mechanical impact. Encapsulation is typically achieved by employing a transfer molding process to encase the electrical device in a thermoset body. During the process, the polymeric material must be in a liquid state to allow it to flow around and under various components, and subsequently solidify by cross-linking. In order to allow the polymeric material to flow around and completely encapsulate the components without damaging fragile wiring and interconnections frequently used in electronic devices, the polymeric material must have a relatively low viscosity (e.g., typically less than 200 poise).
Epoxy resin systems have been most frequently employed for encapsulating, overmolding and/or underfilling electronic devices such as printed circuit boards. There are several well-recognized disadvantages with the use of these conventional epoxy resin transfer molding processes. First, care must be taken to properly monitor and store the epoxy resins to prevent an increase in viscosity that could cause excessive forces to be exerted on fine wiring and other fragile interconnections resulting in destruction and scrapping of the electronic component. Another problem is that conventional resin transfer molding processes involve slow curing to prevent a rapid increase in viscosity as the resin flows around the component, which could result in destruction of the component due to excessive forces being exerted on the fragile wiring and interconnections. As a result of the slow cure rates and long cure times that are typically employed in the known resin transfer molding processes, cycle times are long and production rates are low. Other disadvantages with epoxy transfer molding include the potential for moisture absorption problems and destruction of electronic components due to thermal stresses resulting from the relatively high temperatures and long cure times required.
It has been proposed to overcome many of the disadvantages of resin transfer molding techniques for encapsulating and/or overmolding by employing reaction injection molding (RIM) techniques. RIM processes employ high-pressure impingement mixing and polymerization of two or more different liquids in a closed mold. The chemicals in the different liquids react to form a solid polymeric product. U.S. Pat. No. 6,143,214 discloses the use of a RIM process utilizing a cross-linked RIM system comprising tert-butyl-styrene monomer, divinyl benzene cross-linker, butyl lithium initiator, and 80 percent by weight silica fillers. The process overcomes various problems associated with resin molding transfer processes. Because there is not any reaction until the liquid streams are mixed, storage problems and damage of electronic components due to excessive viscosity may be eliminated. Further, reaction times are very significantly reduced resulting in higher productivity and reduced cost.
However, a problem with both the known RIM and resin transfer molding processes is that the low viscosity employed during the time in which the resin flows around the electronic components allows settling of the inorganic fillers (e.g., silica, glass, etc.) that are generally used to match the coefficient of thermal expansion (CTE) of the encapsulating material to that of the materials used to make the electronic components (e.g., metal oxides and other semi-conductor materials). This settling can result in a significant difference in the CTE at different locations within the encapsulating material, which may be detrimental to the reliability of an electronic component (e.g., a semi-conductor device) that is exposed to significant temperature variations during use or storage.
Another problem with both the known RIM and resin transfer molding processes is that relatively high filler loadings are required. In order to achieve a desirable CTE, the encapsulating resin typically contains from about 80 percent to about 90 percent silica (or comparable inorganic filler) by weight. This high filler level has a deleterious effect on the rheology of the encapsulating material. Specifically, the high filler loading can cause an increase in viscosity that prevents the resin from completely flowing around and completely encapsulating or overmolding the component. The increase in viscosity due to the high filler loading needed to achieve a desirable CTE and provide improved component reliability can also lead to destruction of the component during encapsulation due to excessive forces being exerted on fine wires and/or inner connections during flow of the encapsulating material.
Thus, there is a need for improved encapsulated, overmolded, and/or underfilled electrical components in which desirable rheology allows complete encapsulation, overmolding, and/or underfilling, and wherein the solidified encapsulating and/or overmolding composite material has a desirable CTE and a uniform CTE (i.e., substantially free of a CTE gradient).
Another problem with conventional thermoset encapsulating materials used to protect electronic components is that the thermoset materials are generally expensive, difficult to process, and brittle. As a result, thermoset encapsulated components are expensive and are not as well protected against shock and/or impact as may be desired. Thus, there is also a need for improved encapsulated, overmolded, and/or underfilled electrical components in which the encapsulating material is less expensive, easier to process, and tougher than the thermoset resins that have typically been employed.